Method of controlling optical characteristic of optical element and optical device

ABSTRACT

An optical element is formed by using a plurality of pillar-shaped members arranged periodically and a pair of support members arranged perpendicularly relative to the direction of arrangement of the pillar-shaped members so as to sandwich the pillar-shaped members. The optical element showing a periodic structure of periodic distribution of refractive index. An optical characteristic of the optical element is modulated by applying force to the optical element in a direction perpendicular to the direction of arrangement of the pillar-shaped members and changing the diameter of the pillar-shaped members.

TECHNICAL FIELD

The present invention relates to a method of controlling an opticalcharacteristic of an optical element and also to an optical device.

BACKGROUND ART

“Photonic crystal” is an artificial novel crystal that has been proposedin recent years and is attracting attention ever since (E. Yablonovitch,Phys. Rev. Lett., 58 (1987) 2059-2062). It can be obtained byperiodically arranging substances having different refractive indexes atintervals substantially equal to the optical wavelength. Research anddevelopment efforts are being paid to produce optical elements out ofsuch a crystal substance because it has peculiar optical characteristicsincluding those of showing a photonic band gap and having apparentlyabnormal refractive indexes that are attributable to its so-calledphotonic band structure resembling to the band structure of asemiconductor and can be engineered artificially in terms of structureand scale.

Active type optical elements are among such optical elements that areworthy of paying attention. An active type optical element is an elementat least one of whose optical characteristics can be externally andactively controlled not only in the design stage but also while it isbeing used. Expected applications of active type optical elementsinclude variable filters, optical switches and many other opticaldevices.

Japanese Patent Application Laid-Open No. H10-253829 proposes a methodof arranging an actuator at the periphery of a fiber diffraction gratingand expanding/contracting it to apply tensile force to the fiber so asto control the distribution of refractive indexes in the fiber.

Japanese Patent Application Laid-Open No. 2001-091911 proposes a methodof introducing a substance such as a piezoelectric element, with whichthe refractive index and the transmittivity can be controlledexternally, into photonic crystal and disturbing the periodicity ofcrystal by utilizing expansion/contraction or changes in thecharacteristics of the substance.

W002/27384 proposes a method of controlling the lattice spacing ofphotonic crystal by applying pressure externally to it.

However, these known techniques are accompanied by drawbacks as listedbelow.

Firstly, the method of expanding/contracting an optical fiber requires amember for transmitting force to the fiber in addition to a member forgenerating expanding/contracting force that may be a piezoelectricelement because the one-dimensional periodic structure arranged in thedirection of incident light needs to be changed. Then, there arises aproblem that the precision of controlling the lattice spacing depends onthe material, the arrangement and the connection of the forcetransmitting member. Additionally, while apparently abnormal refractiveindexes as described above appear in a periodic structure that is two orhigher dimensions, the overall arrangement becomes complex when forceneeds to be applied in two or more than two directions by way of a forcetransmitting member.

The method of introducing a means for disturbing structure of photoniccrystal into the latter requires a large number of processing steps andthe scope of materials that can feasibly be used is limited.

Finally, the method of externally applying pressure to a photoniccrystal to change the crystal structure thereof is accompanied by aproblem that, as pressure is applied to a photonic crystal showing aperiodic distribution of refractive indexes as shown in FIG. 5 (verticalparallel lines indicate the period in FIG. 5) to deform the crystal,quantities of deformation are accumulated from the center toward theperiphery to remarkably distort the entire crystal as shown in FIG. 6.Additionally, the characteristics of the crystal can appear unevenly atdifferent sites in the crystal. Furthermore, changes in the distributionof refractive indexes within a period, those in the period and those inthe phase coexist to make it difficult to control the crystal structure.

In view of the above identified circumstances, it is therefore theobject of the present invention to provide a method of controlling anoptical characteristic of an optical element that can improve thedimensional precision of the entire optical element, unify the opticalcharacteristics and enhance the degree of freedom of choosing materialsand also an optical device that can be used with the method.

DISCLOSURE OF THE INVENTION

The present invention provides a method of controlling an opticalcharacteristic of an optical element and an optical device that haverespective configurations as described below.

In an aspect of the invention, there is provided an optical devicecomprising an optical element formed by using a plurality ofpillar-shaped members arranged periodically and a pair of supportmembers arranged perpendicularly relative to the direction ofarrangement of the pillar-shaped members so as to sandwich thepillar-shaped members, the optical element showing a periodic structureof periodic distribution of refractive index, and a means for applyingforce to the pillar-shaped members by way of the support members in adirection perpendicular to the direction of arrangement of thepillar-shaped members.

In another aspect of the invention, there is provided a method ofmodulating an optical characteristic of an optical element formed byusing a plurality of pillar-shaped members arranged periodically and apair of support members arranged perpendicularly relative to thedirection of arrangement of the pillar-shaped members so as to sandwichthe pillar-shaped members, the optical element showing a periodicstructure of periodic distribution of refractive indexes, wherein theoptical characteristic is modulated by applying force to the opticalelement in a direction perpendicular to the direction of arrangement ofthe pillar-shaped members and changing the diameter of the pillar-shapedmembers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of optical deviceaccording to the invention, showing the configuration thereof;

FIG. 2 is a schematic illustration of the optical element of theembodiment of FIG. 1;

FIG. 3 is a schematic illustration of the operation of regulating theembodiment of FIG. 1;

FIG. 4 is a schematic view of the embodiment as seen along the normal tothe surface of the substrate to illustrate the periodic structurethereof;

FIG. 5 is a schematic illustration of one of the problems of the priorart; and

FIG. 6 is a schematic illustration of other one of the problems of theprior art.

BEST MODE FOR CARRYING OUT THE INVENTION

In an embodiment of the invention, a two-dimensional photonic crystal isformed by periodically arranging independent pillar-shaped members thatare made of a deformable material and sandwiching the pillar-shapedmembers between a pair of support members. The support members are rigidand their deformations are negligible. The pillar-shaped members aresandwiched between and rigidly secured to the support members. Morespecifically, the pillar-shaped members are rigidly secured to thesupport members by means of an adhesive at the opposite ends thereof.Alternatively, the adhesive may be used only at one of the supportmembers or the support members may support the pillar-shaped membersonly by pressure without using any adhesive. However, when the supportmembers support the pillar-shaped members only by pressure, onlypressure can be transmitted to the pillar-shaped members as externalforce by way of the support members and no tensile force can betransmitted to them.

An actuator for applying external mechanical force to the surfaces ofthe support members is fixed to the crystal. The actuator is driven toapply force to the pillar-shaped members perpendicularly by way of thesupport members. As a result, the height of the pillar-shaped memberschanges and, at the same time, the diameter and the cross section of thepillar-shaped members also change at a middle part between the top andthe bottom thereof. More specifically, the cross section of thepillar-shaped members increases at a middle part thereof when pressureis applied to them from the top and bottom support members, whereas itdecreases when tensile force is applied to them. The ratio of the changein the cross section caused by the applied force varies depending on theheight and the cross sectional profile of the pillar-shaped members aswell as on Young's modulus and Poisson's ratio that are specific to thematerial of the pillar-shaped members.

The support members are made of a material having a rigiditysufficiently greater than the pillar-shaped members. Therefore, theforce is used effectively to deform the pillar-shaped members. Further,the pillar-shaped members are secured to the support members so that theperiod of arrangement may not be changed.

The change in the cross section changes the distribution of refractiveindexes in a period that are arranged periodically to by turn change thephotonic band structure. As a result, it is possible to regulate anoptical characteristic of the photonic crystal that may relates toreflection or refraction of light.

With the above described arrangement, the optical characteristic can beregulated substantially only by changing the diameter of thepillar-shaped members. This means that, even when the pillar-shapedmembers are arranged over a large area, they can be deformed uniformly,while maintaining the overall dimensional precision. In other words, thecharacteristics of the entire crystal can be unified after a regulatingoperation.

Additionally, with the above described arrangement, the members arepractically not subjected to restrictions and the designer of such aphotonic crystal can enjoy an enhanced degree of freedom for selectingthe materials. In other words, the designer can select appropriatematerials that can simplify the manufacturing process.

When the support members are made of a piezoelectric material, they canbe used as actuator so that force can be directly applied to thepillar-shaped members.

The pillar-shaped members are arranged apart from each other. The voidspace between the pillar-shaped members may be filled with some othermaterials not preventing the deformation of the pillar-shaped members,including gases, liquids, gel-like materials or flexible resins.

Thus, according to the invention, it is possible to provide a method ofcontrolling an optical characteristic of an optical element that canimprove the dimensional precision of the entire optical element, unifythe optical characteristics and enhance the degree of freedom ofchoosing materials and also an optical device that can be used with themethod.

Now, the present invention will be described by referring to theaccompanying drawings that illustrate a preferred embodiment of theinvention.

FIG. 1 is a schematic illustration of an embodiment of optical deviceaccording to the invention, showing the configuration thereof. Referringto FIG. 1, an actuator 101 formed by using a piezoelectric element and aphotonic crystal 105 are laid one on the other and contained in acabinet 106. The actuator 101 operates to expand and compress thephotonic crystal 105 in the opposite directions of the normal to thesurfaces of the substrates 104. The cabinet 106 is provided with twoholes arranged at a pair of opposite sides of the photonic crystal 105in order to secure a light path through the photonic crystal 105.

A laser beam that comes from an optical fiber 110 and enters the elementthrough one of the holes is transmitted through the photonic crystal 105and goes out through the other hole before it strikes light receivingelement 109. The light receiving element 109 comprises a photodiode andis adapted to transmit a signal representing the intensity of incidentlight to a control circuit 107. The control circuit 107 computationallydetermines a control quantity in such a way that the laser beam entersthe light receiving element 109 with a specified intensity and feeds itto drive circuit 108, which drives actuator 101.

The photonic crystal 105 may typically be prepared in a manner asdescribed below. Firstly, a multilayer film is formed by deposition on asubstrates 104 that is made of Si to produce a reflection film 102 forthe purpose of improve the transmittivity. Then, PMMA(polymethylmethacrylate) is applied onto the reflection film 102 and aperiodic structure 103 is formed by X-ray lithography. FIG. 4 is aschematic view as seen along the normal to the surface of the substrate104 to illustrate the periodic structure thereof. Each of thepillar-shaped members of the periodic structure 103 is isolated and atwo-dimensional periodic structure is formed on the surface plane of thereflection film 102 that is parallel to the surface of the substrate104.

Then, another substrate 104, which is made of Si and on which areflection film 102 is formed, is prepared and the two substrates 104are put together with the reflection films 102 facing the periodicstructure 103 of PMMA. In this way, a photonic crystal 105 in which aperiodic structure 103 is sandwiched between a pair of reflection films102 that are arranged in parallel with each other and then between apair of substrates 104 is formed.

FIG. 3 illustrates a process of applying external force to the photoniccrystal 105 of FIG. 2 along the normal to the substrates 104. Each ofthe pillar-shaped members of the periodic structure 103 is deformed tochange its diameter and the optical characteristics of the photoniccrystal 105. Since the individual pillar-shaped members are isolated, noaccumulation of quantities of deformation takes place in the plane ofarrangement of the periodic structure 103 so that the crystal is notdeformed as a whole. Note that the materials of the reflection films102, the periodic structure 103, the substrates 104 and so on are notlimited to those described above. In other words, appropriate materialsmay be selected from the viewpoint of physical properties includingrefractive index and Young's modulus, adaptability to the manufacturingprocess and operating environment including temperature and humidity.

While an piezoelectric element is used as actuator 101 in thisembodiment, some other drive mechanism such as a feed screw mechanism ora voice coil may alternatively be used.

Additionally, while the present invention is applied to regulate thetransmittivity as optical characteristic, it may be needless to say thatit is also applicable to other optical characteristics such asreflectivity.

1. An optical device comprising: an optical element formed by using aplurality of pillar-shaped members arranged periodically and a pair ofsupport members arranged perpendicularly relative to the direction ofarrangement of the pillar-shaped members so as to sandwich thepillar-shaped members, said optical element showing a periodic structureof periodic distribution of refractive index; and means for applyingforce to the pillar-shaped members by way of the support members in adirection perpendicular to the direction of arrangement of thepillar-shaped members.
 2. The optical device according to claim 1,wherein the force applied to the pillar-shaped members in a directionperpendicular to the direction of arrangement changes not only theheight but also the diameter of said pillar-shaped members.
 3. Theoptical device according to claim 1, wherein said support members aremade of a material having a rigidity greater than said pillar-shapedmembers.
 4. The optical device according to claim 1, wherein theperiodic structure does not change its period when said pillar-shapedmembers are deformed.
 5. The optical device according to claim 1,wherein at least one of said pair of support members is a piezoelectricelement.
 6. The optical device according to claim 1, wherein areflection layer is formed on each of said support members at the sidefacing said pillar-shaped members.
 7. The optical device according toclaim 1, wherein at least one of said pair of support members is fixedto a piezoelectric element and said means for applying mechanical forcecomprises electrodes arranged on the piezoelectric element and a circuitfor applying a voltage to the electrodes.
 8. A method of modulating anoptical characteristic of an optical element formed by using a pluralityof pillar-shaped members arranged periodically and a pair of supportmembers arranged perpendicularly relative to the direction ofarrangement of the pillar-shaped members so as to sandwich thepillar-shaped members, said optical element showing a periodic structureof periodic distribution of refractive indexes, wherein the opticalcharacteristic is modulated by applying force to the optical element ina direction perpendicular to the direction of arrangement of thepillar-shaped members and changing the diameter of the pillar-shapedmembers.